Step 1: No single objective criterion:
A nation cannot be defined by any one objective marker; common language, religion or ethnicity is neither necessary nor sufficient, since many nations (like India) are multi-lingual and multi-religious, while some linguistic/religious groups span several different nations.
Step 2: Subjective, contested nature:
A nation is ultimately a subjectively 'imagined' political community, people who feel they belong together, so its membership and boundaries are shaped by shared consciousness and history rather than by a fixed, measurable rule.
Step 3: Political and historical construction:
Because national identity is historically and politically constructed, and can change over time or be contested by different groups within the same territory, arriving at one universal definition remains difficult.
Final Answer:
Defining a nation is difficult because no single objective feature (language, religion, ethnicity) reliably marks its boundary; it is instead a subjectively felt, historically constructed political community, always open to contestation.